It is desirable to operate gasoline engines in a lean-burn mode, where the A/F ratio is higher than stoichiometry, in order to improve fuel economy. The difficulty in treating such oxidizing engine exhaust gases is to convert particularly the nitrogen oxides (NOx) generated in the gases along with the carbon monoxide and hydrocarbons. The first must be reduced while the latter two are oxidized. Lean-burn catalysts to carry out this simultaneous conversion have been less than commercially successful. Of current interest are NOx absorbents often called lean-burn NOx traps. These materials are able to absorb nitrogen oxides from the exhaust gases during lean-burn operation and then later release them when the oxygen concentration in the exhaust gases is reduced. For example, when the A/F ratio is made rich or stoichiometric. Conventional NOx absorbents are alkaline earth metals like barium with a precious metal catalyst like platinum carried on alumina. In a recent application concurrently owned application Ser. No. 09/184,146 filed Nov. 2, 1998 and entitled "Use of Sol-Gel Processed Alumina-based Metal Oxides for Absorbing Nitrogen Oxides in Oxidizing Exhaust Gas", we disclose the unexpected advantage of using sol-gel processed materials which include alkali metal, alkaline earth metal, or their mixture in an alumina matrix. These materials are found to be more resistant to sulfur poisoning than conventional alumina materials.
We have now found that the NOx absorption of sol-gel processed materials, as those of the above referenced application, can be significantly improved by making mesoporous molecular sieve aluminum oxide materials using a surfactant as a template during the sol-gel processing.
Aluminum oxide molecular sieves which incorporate cerium and lanthanum are disclosed in "Heterometallic Alkoxides as Precursors for the Incorporation of Lanthanides in Alumina Molecular Sieves" in Research Disclosure, June 1997, 39825. In "Rare Earth Stabilization of Mesoporous Alumina Molecular Sieves Assembled Through an N.degree.I.degree. Pathway", Chem. Commun., 1998, Zhang and Pinnavaia disclose incorporation of Ce.sup.+3 or La.sup.+3 ions in the alumina to improve thermal stability. In "Synthesis of Porous Yttrium Aluminium Oxide Templated by Dodecyl Sulfate Assemblies", Chem. Comm., 1998, Yada et. al. disclose yttrium aluminum oxide with a hexagonal structure. In "High Catalytic Efficiency of Transition Metal Complexes Encapsulated In A Cubic Mesoporous Phase" Rao et.al. disclose preparation of an alumina-silica molecular sieve (Al-MCM-48) employing cetyltrimethylammonium bromide. None of these references disclose, however, forming molecular sieves of the present invention.
In U.S. application Ser. No. 09/134,992 filed Aug. 17, 1998 and entitled "NOx Trap Catalyst For Lean Burn Engines" commonly assigned with the present invention, a sol-gel oxide material is disclosed useful for NOx absorption. It comprises oxides of aluminum, magnesium and zirconium.